Place bags side by side in a large pot. Add cold water to cover by 2". Heat water over medium heat until an instant-read thermometer registers 150°. Turn off heat, cover, and poach chicken for 50 minutes. Transfer bags to a large bowl of ice water to cool, about 15 minutes. Remove chicken from bags; pat dry.

Preheat oven to 450°. Heat vegetable oil in a large cast-iron skillet over high heat. Add chicken halves, skin side down, so chicken sits against sides of pan. Cook, moving chicken occasionally for even cooking, until skin is browned all over. Flip chicken and transfer skillet to oven. Roast until an instant-read thermometer inserted into thickest part of thigh registers 165°, about 15 minutes. Let rest for 10 minutes.